The present invention generally relates to a method of suitably texturing at least part of the exposed surface(s) of an article with a dual surface structure which is inherently water repellant including, but not limited to, organic polymers optionally containing a variety of additives and/or fillers, by employing amorphous and/or fine-grained metallic embossing dies or molds exhibiting a relief pattern containing a dual surface structure.
Water repellant (hydrophobic), super-hydrophobic and self-cleaning surfaces are desired in numerous applications involving, at least at times, exposure to the atmosphere or water. According to the prior art, known super-hydrophobic surfaces (contact angle for water greater than or equal to 150°) which are self-cleaning are created by introducing artificial surface structures including elevations and/or depressions in a smooth surface of an inherently hydrophobic material (contact angle for water greater than 90°), for example, organic polymers or coatings. Suitable distances between the elevations and/or depressions are reported to be in the range of from 5 μm to 200 μm, and the heights and/or depths of the elevations and/or depressions are in the range of from 5 μm to 100 μm.
The prior art describes various means of increasing the water repellent properties of hydrophobic surfaces by roughening.
U.S. Pat. No. 3,354,022 describes water repellent surfaces having an intrinsic advancing water contact angle of more than 90° and an intrinsic receding water contact angle of at least 75° by creating a micro rough structure with elevations and depressions in a hydrophobic material. The high and low portions have an average distance of not more than 1,000 microns, an average height of high portions of at least 0.5 times the average distance between them. The air content is at least 60% and, in particular, fluorine containing polymers are disclosed as the hydrophobic material. The water repellent surfaces are created using an embossing die made of hollow polymer fibers. Unfortunately, such embossing dies are cumbersome to produce and have a limited durability.
U.S. Pat. No. 6,660,363 describes self-cleaning surfaces of objects made of hydrophobic polymers or permanently hydrophobized materials which have an artificial surface structure of elevations and depressions. The distances between the elevations are in the range of from 5 μm to 200 μm, and the heights of the elevations are in the range of from 5 μm to 100 μm. At least the elevations consist of hydrophobic polymers or permanently hydrophobized materials. The elevations cannot be wetted by water or by water containing detergents by attaching PTFE particles (7 micron in diameter) to a polymer adhesive film containing surface and curing the structure or by using a fine mesh screen to emboss a polymer surface by hot pressing. According to the '363 patent, such surfaces are produced by application of a dispersion of powder particles of an inert material in a siloxane solution, and subsequent curing the siloxane solution to form a polysiloxane. Unfortunately, the structure forming particles do not adhere well to the surface, are cumbersome to produce, and have a limited durability.
U.S. Patent Publication No. 2003/0187170 discloses a process for producing nanostructured and microstructured polymer films by guiding the polymer through a gap formed by a suitably patterned roll and a means which develops an opposing pressure so that the polymer film is deformed and shaped in accordance with the relief pattern. No information is provided in the '170 publication to substantiate the nanostructured features disclosed or to demonstrate the wetting behavior of the embossed polymer films.
U.S. Pat. No. 6,764,745 describes a structural member in which high water-repellency can be obtained by forming appropriate irregularities on the external surface. The irregularities comprise protrusion portions of uniform height and shaped as prisms and which are subsequently coated with a water repellent film of PTFE or fluoroslkylsilane. The surface features termed “irregularities” have such dimensions that a water droplet cannot fall into the air-filled recesses. This approach requires multiple materials and/or layers and a final topcoat to render the article superhydrophobic.
U.S. Pat. No. 6,872,441 describes glass, ceramic and metal substrates with at least one self-cleaning surface comprising a layer with a micro-rough surface structure which is arranged on the substrate and made at least partly hydrophobic. The layer contains a glass flux and structure-forming particles with a mean particle diameter within the 0.1 to 50 micron range. The micro-rough surface structure has a ratio of mean profile height to mean distance between adjacent profile tips within the 0.3 to 10 micron range. The surface layer is produced by coating the substrate with a composition containing a glass flux and structure-forming particles, and the layer is burnt in and made hydrophobic. This approach requires multiple materials and/or layers and a final topcoat to render the article superhydrophobic and is rather complex.
U.S. Ser. No. 12/785,650, entitled “METALLIC ARTICLES WITH HYDROPHOBIC SURFACES”, which has a common assignee and is filed concurrently with the present application, describes articles having exposed metallic surfaces comprising durable, fine-grained and/or amorphous microstructures which, at least in part, are rendered water repellant by suitably texturing and/or roughening the surface to increase the contact angle for fluids including water, thus reducing the wetting behavior of the surface, reducing corrosion and enabling efficient cleaning and drying.
The prior art also addresses the fabrication of fine-grained and/or amorphous metallic coatings and articles for a variety of applications.
U.S. Pat. No. 3,303,111 discloses amorphous nickel phosphorus (Ni—P) and/or cobalt phosphorus (Co—P) coatings using electroless deposition.
U.S. Pat. No. 5,389,226 discloses amorphous and microcrystalline electrodeposited nickel-tungsten (Ni—W) coatings of high hardness, wear and corrosion resistance and low residual stress to avoid cracking and lifting of the coating from the substrate.
U.S. Pat. No. 5,032,464 discloses smooth ductile alloys of a transition metal and phosphorus, particularly nickel phosphorus (Ni—P) with high ductility (up to 10%) produced by electrodeposition.
U.S. Pat. No. 5,288,344 describes beryllium (Be)-bearing alloys which form amorphous metallic glasses upon cooling below the glass transition temperature at a cooling rate appreciably less than 106 K/s.
U.S. Pat. No. 7,575,040 describes a process for continuous casting amorphous metal sheets by stabilizing the molten alloy at casting temperature, introducing the alloy onto a moving casting body, and quenching the molten alloy to solidify it.
U.S. Pat. No. 5,352,266 and U.S. Pat. No. 5,433,797, which are assigned to the same assignee, describe a process for producing nanocrystalline materials, particularly nanocrystalline nickel. The nanocrystalline material is electrodeposited onto the cathode in an aqueous acidic electrolytic cell by application of a pulsed current.
U.S. Patent Publication No. 2005/0205425 and DE 10228323, both being assigned to the same assignee as the present application, disclose a process for forming coatings, layers or freestanding deposits of nanocrystalline metals, metal alloys or metal matrix composites. The process employs tank plating, drum plating or selective plating processes using aqueous electrolytes and optionally a non-stationary anode or cathode. Nanocrystalline metal matrix composites are disclosed as well.
U.S. Patent Publication No. 2009/0159451, which has a common assignee as the present application, discloses graded and/or layered, variable property electrodeposits of fine-grained and amorphous metallic materials, optionally containing solid particulates.
U.S. Ser. No. 12/548,750, assigned to the same assignee as the present application, discloses fine-gained and amorphous metallic materials comprising cobalt (Co) of high strength, ductility and fatigue resistance.
U.S. Ser. No. 12/785,524, which is a continuation-in part of U.S. Ser. No. 12/476,455, entitled “METAL CLAD POLYMER ARTICLE”, and is filed concurrently with the present application, discloses metal-clad polymer articles comprising polymeric materials having fine-grained (average grain-size of 2 nm to 5,000 nm) or amorphous metallic materials of enhanced pull-off strength between the metallic material and the polymer which are optionally wetproofed.
DE 10108893 describes the galvanic synthesis of fine-grained group II to group V metals, their alloys and their semiconductors compounds using ionic liquid or molten salt electrolytes.
U.S. Pat. No. 5,302,414 describes a cold gas-dynamic spraying method for applying a coating to an article by introducing metal or metal alloy powders, polymer powders or mixture thereof into a gas stream. The gas and particles, which form a supersonic jet having a velocity of from about 300 msec to about 1,200 msec, are directed against a suitable substrate to provide a coating thereon.
U.S. Pat. No. 6,895,795 describes a method of processing a billet of metallic material in a continuous manner to produce severe plastic deformation. The billet is moved through a series of dies in one operation to produce a billet with a refined grain structure.
U.S. Pat. No. 5,620,537 describes a method of superplastic extrusion for fabricating complex-shaped, high strength metal alloy components by carefully controlling strain rate and temperature to retain an ultra-fine grained microstructure. A high strength, heat treatable metal alloy is first processed, such as by equal channel angular extrusion (ECAE), to have a uniform, equiaxed, ultra-fine grain size in thick section billet form.
Thus, the prior art teaches that, in order to substantially raise the contact angle for water, surface features need to be introduced to the exposed surface of an inherently non-wetting material as illustrated in numerous naturally occurring structures. The material has to be inherently hydrophobic and has to contain a patterned surface comprising suitable depressions and/or elevations as observed in lotus or aspen leaves, rose petals, etc.
Numerous attempts have been made to replicate nature as noted above to achieve super-hydrophobic and/or self-cleaning properties in man-made articles. As indicated previously, U.S. Patent Publication No. 2003/0187170 describes embossed polymer surfaces with nano-sized and micro-sized structures by shaping/patterning the polymer surface with appropriate processes including injection molding, die embossing, or rotary embossing. The relief pattern on the forming tool can be created by sandblasting, etching, laser ablation, lithographic techniques, offset printing, electroplating techniques, LIGA and/or erosion. The '170 publication guides towards the employ of LIGA to create forming tools with dimensions in the nanometer range (100 nm to 300 nm depth). The use of galvanoforming to generate the forming tool by plating onto a negative hollow mold of plaster, wax etc. is disclosed as well. The working examples illustrate the production of forming tools using LIGA with the smallest dimension (depth) being 1,000 nm (1 micron). None of the working examples illustrate the generation of nano-structured features overlaying a micro-structured features as set forth in the present application, i.e., a dual structure composed of a primary features in the 1 μm to 1,000 μm range (i.e., a primary structure) overlayed by a secondary structure with ultra-fine features in the range of between 1 nm and 100 nm. The only plausible interpretation of the '170 publication suggests that the spacing and/or height/depth of the micro-structures themselves can have a dimension in the nanometer range. However, there is no teaching or suggestion in the '170 publication of a dual microstructure including ultra-fine features equal to or less than 100 nm embedded in and overlaying a surface topography with macro-surface structures equal to or greater than 1 micron of the present application.
The '170 publication discloses the use of metallic or a polymeric materials as an imprinting tool. The '170 publication, however, makes no distinction with respect to the material microstructure and all information provided suggests to the person skilled in the art that, in the case of using metallic imprinting rolls, conventional metallic materials such as steel with a coarse grained microstructure (average grain size greater than 30 microns) are being employed. Similarly, the general electroforming techniques disclosed would produce coarse-grained deposits. As highlighted, the only dimension in the nanometer range disclosed by the '170 publication is the depth of the structure which ranges over six (6) orders of magnitude, from 10 nm to 10,000,000 nm (10,000 μm). Notably, no single contact angle measurement, tilt angle measurement or any evidence of super-hydrophobic properties using the process disclosed, are provided in the '170 publication.